A surface waveguide is a light-guiding element, much like an optical fiber, which is formed on the surface of a rigid substrate. Although constrained to the substrate, the surface waveguide can traverse any path in the plane of the surface including curves, loops, and relatively sharp corners, subject to design constraints based on the optical properties of the materials used to form the surface waveguide. Surface waveguides are widely used in many applications including telecommunications, chemical sensing, and force sensing.
A surface waveguide is characterized as having a central region or “core” and a surrounding “cladding.” An optical signal travels through a surface waveguide as an optical mode propagating through the core. The optical signal is confined to the core by the cladding. The guiding property of a surface waveguide stems from a difference in the speed at which light travels in the material of the core versus the material of the cladding. Light travels at different speeds in different materials, and every material has a “refractive index,” n, which is a measure of the speed of light in that material versus the speed of light in vacuum.
When light traveling in one material hits a boundary of a new material with a different refractive index, the light will reflect off the boundary, be bent (i.e., refract, as in a prism), pass through into the new material, or some combination thereof. The amount of light that is reflected, as compared to the amount of light that passes into the new material depends on the relative refractive indices of the materials and the angle at which the light hits the boundary. In the case of a surface waveguide, the refractive index of the cladding is typically only slightly lower than the refractive index of the core. Furthermore, light traveling in the core travels in a direction that is nearly parallel to the interface between the core and the cladding. Therefore, when light traveling in the core hits the boundary with the cladding, nearly all of the light bounces back into the core in the same way that a flat stone bounces off the surface of a pond when it's skipped. The propagating mode is, therefore, effectively confined to the core.
Surface waveguides can be fabricated in various forms including slab waveguides, ridge waveguides, and stripe waveguides. A slab waveguide comprises a planar thin film of optical core material sandwiched between two planar thin films of cladding material. The cladding material above and below the core slab confine the propagating optical mode in the vertical direction, but not in the horizontal direction.
A ridge waveguide is similar to a slab waveguide, but in addition to a slab region also includes a protruding ridge of material through which an optical mode propagates. The lateral and vertical structure substantially confines the mode in both dimensions, except where the ridge meets the slab. It is possible that a mode can exist in the slab area outside the ridge portion.
A stripe waveguide is a ridge waveguide wherein the slab layer has been etched completely away except in the ridge area. Since there is no core material anywhere except in the stripe, an optical mode will propagate only in the core.
The material used for the core and cladding are chosen based on their relative optical properties. Surface waveguides have been formed from a variety of materials, including different types of glasses (e.g, silicon dioxide, boro-phosphosilicate glass, phosphosilicate glass, etc.), silicon nitrides, silicon oxy-nitrides, gallium arsenide, indium phosphide, silicon, and lithium niobate. These materials are used in combination to enhance the guiding ability of the surface waveguide. The most common surface-waveguide material is glass, wherein core and cladding glasses are doped with different impurities or different concentrations of the same impurity in order to make the refractive index of the cladding glass slightly lower than the refractive index of the core glass to provide light-guiding capability.
The surface waveguide is formed by successively depositing and patterning thin films of optical materials onto the surface of a substrate. Low pressure chemical vapor deposition (LPCVD) is a common method of forming the thin-film layers. In an LPCVD system, the glass is deposited onto the surface of a silicon wafer in high temperature furnaces into which different precursor gasses are injected, resulting in a chemical reaction that deposits glass on the silicon wafer surface.
The deposition conditions and the types of gasses can be changed to produce various glass types having different optical and mechanical properties. In addition, deposition conditions and precursor-gas type affect the way the resulting film covers features on the surface. Certain gas mixtures or materials are known to result in what are called “conformal” layers, wherein the thickness of the deposited film is nearly uniform over underlying structure. Materials that deposit conformally during LPCVD include polycrystalline silicon, silicon dioxide deposited using a precurser gas of tetraethylorthosilicate, (this type of silicon dioxide will hereinafter be referred to as “TEOS”), and stoichiometric silicon nitride (silicon nitride that has the exact formulation of three silicon to every four nitrogen atoms: Si3N4).
Unfortunately, conformally- and nonconformally-deposited thin films exhibit inherent residual stress due to the deposition process itself and due to the material characteristics of the films, such as differences in thermal expansion coefficients. If the underlying films or substrate include surface topography, then such stresses can be directionally dependent as well.
The refractive index of a pure, unstrained, non-crystalline material is always exactly the same at a specific temperature. For example, pure, unstrained silicon dioxide (SiO2) has a refractive index of exactly 1.46 at room temperature (300 K). By adding an impurity (e.g., phosphorous, etc.) and controlling its concentration, however, the refractive index of silicon dioxide can be altered. Different concentrations or impurities are used to vary the refractive index as desired within materially-defined limits. The refractive index of most materials can be controlled in the same way, and the ability to control the refractive index in this manner is exploited in surface-waveguide and optical-fiber technology alike.
There are other factors that will cause the refractive index of a material to change. Some of these factors include a variation in temperature, internal stress caused by the presence of impurities, and mechanical strain. Often, these factors are directional, such as a thermal gradient through the thickness of a layer, differences in the vertical and horizontal dimensions of a waveguide, or a mechanical force applied in only one plane. As a consequence, the refractive index of such a material becomes directionally dependent. That is, there are two refractive indices for the material. Such a material is said to be birefringent.
A surface waveguide supports the propagation of light that has two directionally-dependent components, referred to as polarization modes TE and TM. These polarization modes are essentially flat sinusoidal waves that are orthogonal (i.e., physically oriented at right angles to each other), with the TE mode being horizontally oriented and the TM mode being vertically oriented. Due to directionally-dependent stress resulting from thin-film deposition, the polarization modes in a typical surface waveguide see significantly different refractive indices. This is known as “modal birefringence”, and is quantified as nTE-nTM. Modal birefringence is particularly prevalent in ridge and stripe waveguides, wherein the core material is patterned to have a roughly square cross-section and subsequently over-coated with a conformal layer of cladding material, exacerbating stress and stress gradients present in the different layers.
Modal birefringence has thus far limited the utility of surface waveguide structures. For most applications using optical fibers or surface waveguides, it is necessary that the two polarization modes travel through the surface waveguide at the same speed. Divergence of the modes as they travel can lead to serious system complications for many applications. In a communications system, for example, it is well understood that dire consequences result from differences in the received optical power of the polarization modes, or the time at which the polarization modes are received, or when optical power transfers from one polarization mode to the other. It is highly desirable therefore, to form surface waveguides having low modal birefringence.